User guide

Important Swarming concepts are Tasks and Bots. A task is a step that has inputs and generates outputs. A bot can run tasks. Simple, right?


There are 3 important classes of data:

  • Task properties defines inputs (files, command), a description of the dimensions a bot must have to run it, timeouts. It's the functional part.
    • It is the description of what requester wants to do. Properties act as the key in the cache of finished results (see Idempotency).
  • Task request defines the particular attempt to execute a task. It contains task properties (what to execute), and accompanying metadata for bookkeeping tweaking (who executes, when, why, whether it is ok to reuse existing cached result, etc).
  • Task result defines which bots ran it, timestamps, stdout, outputs if any, performance metadata, logs.

Task request (and task properties as part of it) are set when the task is created and do not change.

A task is referenced to via a task ID. It looks like an hex number but should be treated as a string.


A Swarming task is conceptually a function that has some input and generates outputs. The process can be simplified as F(i, c, b) where F() is the Swarming task, i is the input files, c the command execution environment, b the bot selection (dimensions) description.

Inputs can be a mix of all 4 of:

  • Isolated tree: optionally includes a command. It's essentially a pointer (SHA1 digest) to a root of a Merkle tree-like structure that stores files for the task.
  • CIPD package(s): archives to be mapped inside the work directory. This leverages CIPD.
  • Named cache(s): local cache directory(ies) to do incremental work to be mapped in the task working directory. This tells the worker to create directory(ies) to keep even after the task is completed, so files can be reused by some later task that requests the exact same cache. Examples are local git clone caches, incremental build directories, etc. This adds transitivity to the task (non-determinism).
  • Secret bytes. This data is sent to the task via LUCI_CONTEXT but is not otherwise retrievable.

Command execution environment is defined as:

  • Command line:
    • From the isolated file. In this case, it is possible to extend with extra_args to specify arguments appended to the command on a per-task basis. A good use is to shard test runs into multiple subshards.
    • In the task properties. In this case, the actual command is listed as a list.
  • Idempotency flag to signify possibility of reusing previous succeeding results of a task with the exact same task properties.
  • Timeouts:
    • Hard timeout: the maximum allocated time to run the task.
    • I/O timeout: the maximum allowed time between output to stdout or stderr. For example, an I/O timeout of 20 minutes and a Hard timeout of 60 minutes, a task sleeping for 20 minutes will be killed, even if still has allocated time. This catches hung tests faster.
  • Environment variables can be leveraged to create side effects, for example test sharding.

Bot selection is defined as:

  • A list of key:value dimensions. This is the core bot selection mechanism to select which bots are allowed to run the task.

The dimensions are important. For example be clear upfront if you assume an Intel processor, the OS distribution version, e.g. Windows-7-SP1 vs Windows-Vista-SP2.


Idempotency is a mechanism to improve the efficiency of the infrastructure. When a task is requested to the server, and a previous task with the exact same properties had previously succeeded on the Swarming server, previous results can be returned as-is without ever running the task.

Not running anything is always faster than running something. This saves a lot of time and infrastructure usage.

To effectively leverage idempotency, it is important for the inputs files to be as “stable” as possible. For example efforts, see Debian‘s initiated effort reproducible builds and Chromium’s deterministic builds effort.

To enable this feature, a task must be declared as idempotent. This tells the server that this request fits the contract that the task implements a pure function: same inputs always produce same outputs. Results of execution of such tasks can be reused.

For a task to be idempotent, it must depend on nothing else than the task inputs and the declared environment. This means the dimensions uniquely describe the type of bot required; exact OS version, any other important detail that can affect the task output.

Other things of note are:

  • No access to any remote service. This include HTTP(S), DNS lookup, etc. No file can be ‘downloaded’ or ‘uploaded’ by the task. They must be mapped in, content addressed, up front. Results must be inside ${ISOLATED_OUTDIR}.
    • This is also important from a performance PoV since keeps a local content addressed cache.
  • No dependency on the time of the day or any other side-signal.

If any of the rule above does not hold, the task must not be marked as idempotent since it is not reproducible by definition.


The request is the metadata around the task properties requested. This defines:

  • Who: authenticated account to create the task and on behalf of whom. For example, the Commit Queue creates a task on the behalf of a user.
  • When: creation timestamp, expiration delay
    • The expiration delay is the maximum allowed time to wait for a bot to pickup the task. If no bot pickups the task in the allowed timeframe, the task is marked as EXPIRED.
  • What: the task properties. As described in the previous section, this defines all inputs for the task and the hash digest of this data is used as a key in a cache of available results when evaluating idempotent tasks.
  • Why: display name, tags. Tags are used to enable searching for tasks.
  • A numerical priority is associated to the tag between 0 and 255. Lower means most important. This is effectively a FIFO or LIFO queue of tasks.


The result is a collection of:

  • Where the execution process happened, which bot.
  • What are the results of the “function call”. The stdout, the isolated outputs.
  • Metadata like the exit code, the timeout signal if any, timestamps.

Once the task is completed, results become immutable.

The result can also be a non-event: the task wasn't run at all. This results in an EXPIRED task. This happens when there was no bot available to run the task before the expiration delay.

An exceptional event can be BOT_DIED. This means that either the bot was lost while the task ran or that the server had an internal failure during the task execution.


To understand how bot behaves, see This section focuses from the point of view of running a task.

Swarming tasks are normally running a isolated tree directly via

Swarming is designed with inspiration from internal Google test distribution mechanism. As such, it has a few assumptions baked in. A task shall:

  • Open input files as read-only, never for write.
  • Write files only to these two locations:
    • The OS-specific temporary directory, e.g. /tmp or %TEMP% file files that are irrelevant after the task execution.
    • ${ISOLATED_OUTDIR} for files that are the output of this task.

Once the task completed, results are uploaded back and the tree is associated with the task result.


There's 4 ways to interact with Swarming:

  • Swarming web UI. It is primarily a way to view state, not change it. Two exceptions are cancelling a task and retrying a task. The Web UI doesn't provide a way to trigger a new task (ping the author if this feature is desirable).
  • CLI command line tool. There's a python client and eventually a Go client.
    • The luci-py/client repository subdirectory is cloned automatically to client-py so you don‘t have to clone the whole server code if not desired. It’s the same code.
    • Once the Go client code is completed, you'll be able to go get
  • Direct HTTP requests. You can browse the API description online.
    • This is the interface to use to trigger a task via another server.

Web UI

The Web UI has 4 purposes:

  • Present tasks and enable searching per tags and dimensions.
    • Enable canceling or retrying individual task.
  • Present bots and enable querying per dimensions.
  • For administrators:
    • Enable updating and
    • Present a token to bootstrap new bots.
    • View error reports.

Python CLI is the client side script to manage Swarming tasks at the command line.

Warning: This doc is bound to become out of date. Here's one weird trick:

  • help” gives you all the help you need so only a quick overview is given here:

Running a task synchronously

If you just want to run something remotely, you can use the run command. It is going to block until the command has completely run remotely. run --swarming <host> --isolate-server <isolate_host> <isolated|hash>

The <hash> is what archive gave you. See IsolatedUserGuide for more information. A path to a .isolated file will work too.

Running a task asynchronously

Tasks can be triggered asynchronously by using trigger + collect. The general idea is that you trigger all the tests you want to run immediately, then collect the results.


Triggers a task and exits without waiting for it: trigger --swarming <host> --isolate-server <isolate_host> --task <name> <hash>
  • <name> is the name you want to give to the task, like “base_unittests”.
  • <hash> is an .isolated hash.

Run help trigger for more information.

Collecting results

Collects results for a previously triggered task. The results can be collected multiple times without problem until they are expired on the server. This means you can collect again data run from a job triggered via run. collect --swarming <host> <name>

Querying bot states query returns state about the known bots. More APIs will be added, like returning tasks, once needed by someone. In the meantime the web frontend shows these.

More info

The client tools are self-documenting. Use “ help” for more information.


The API is implemented via Cloud Endpoints v1. It can be browsed at:

Until the API is rewritten as proto files, each API struct description can be read at